Is childhood cat ownership a risk factor for schizophrenia later in life?

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Abstract

Two previous studies suggested that childhood cat ownership is a possible risk factor for later developing schizophrenia or other serious mental illness. We therefore used an earlier, large NAMI questionnaire to try and replicate this finding. The results were the same, suggesting that cat ownership in childhood is significantly more common in families in which the child later becomes seriously mentally ill. If true, an explanatory mechanism may be Toxoplasma gondii. We urge our colleagues to try and replicate these findings to clarify whether childhood cat ownership is truly a risk factor for later schizophrenia.

Introduction

In 1995 a study suggested that cat ownership during childhood might be a risk factor for later developing schizophrenia (Torrey and Yolken, 1995). The data came from a 1992 questionnaire filled out by 165 members of the National Alliance for the Mentally Ill (NAMI). At that time, NAMI had approximately 170,000 members and consisted almost exclusively of families in which a family member had been diagnosed with schizophrenia or schizoaffective disorder (approximately two-thirds) or bipolar disorder or severe depression, usually with psychotic features (approximately one-third) (Steinwachs et al., 1992). The question asked whether at any time between the affected family member's birth and age 10 there was a cat living in the house; 84/165 (50.9%) of the cases and 65/165 (39.4%) of the controls responded yes. The controls were obtained by having a family friend, whose child had not developed any mental illness, fill out an identical questionnaire. The cat question was part of two pages of questions covering breastfeeding, developmental milestones, coordination, rheumatoid arthritis, and multiple sclerosis. Although the cat question was statistically significant by itself [chi square = 4.4, p  .036; odds ratio (OR) = 1.60 (1.00–2.53), mean (95% confidence interval)], it was not significant following the application of the Bonferroni correction for the number of questions asked.

Based on these initial findings, a 1997 follow-up survey was undertaken (Torrey, et al., 2000). Trained interviewers at the Survey Research Center, University of Maryland, carried out a 20 minute telephone interview with subjects randomly selected from the NAMI membership list; the refusal rate was 9%. Two controls for each case from families without any serious mental illness were selected from the same telephone exchange as the subjects and matched for age, sex and socioeconomic status; the refusal rate was 28%. The interview focused on the perinatal and preadolescent childhood periods and included 19 major variables, many of which included subsets, asking about the mother's pregnancy, miscarriages/stillbirths, delivery, breast-feeding, developmental milestones and place of residence as well as exposure to dogs, cats, other pets, and farm animals. Regarding family ownership of a cat between the affected person's birth and age 13, 136/262 (51.9%) of cases and 220/522 (42.1%) of controls responded ‘yes’ [chi square = 6.7, p  .01; OR = 1.48 (1.09–2.02)]. Dog ownership was more common among the controls (78.8%) than among the cases (73.1%) although the difference was not statistically significant (p = .09).

Based on these two studies, we decided to analyze cat ownership from a large unpublished NAMI survey carried out in 1982, 10 years before any data on cat ownership and mental illness had been published. The issue is potentially important since cats are carriers, and the definitive hosts, of Toxoplasma gondii. This parasite has been linked to schizophrenia in a meta-analysis of 38 studies of T. gondii antibodies (pooled OR = 2.71) (Torrey et al. 2012). In addition, T. gondii has been shown to produce dopamine, thought to be increased in schizophrenia (Gaskell, et al., 2009). There are also other reasons to suspect that T. gondii may play a role in some cases of schizophrenia (Torrey and Yolken, 2003, Yolken et al., 2009).

Section snippets

Materials and methods

At the NAMI annual convention in 1982, participants whose family member was diagnosed with schizophrenia or schizoaffective disorder were asked to fill out a two-page questionnaire. Questions included conditions of pregnancy, birth weight, childhood illnesses, smallpox vaccination, seizures, age of first referral, illnesses immediately preceding onset, family history of mental illness and rheumatoid arthritis, and cat and dog ownership up to age 17. The question on pets included ages of pet

Results

There were 2125 useable 1982 questionnaires from NAMI families who lived in 46 states and the District of Columbia. The number who owned a cat when the affected person was between birth and 13 was 1075/2125 or 50.6%. This result is remarkably similar to those found in 1992 (50.9%) and 1997 (51.9%) (Table 1). Among the “middle parents” control group from the 1991 AVMA survey, 2065/4087 (42.6%) owned a cat, a result virtually identical to the controls in our 1997 survey (Table 1). The difference

Discussion

Cat ownership in childhood has now been reported in three studies to be significantly more common in families in which the child is later diagnosed with schizophrenia or another serious mental illness, compared to families in which the child is not so diagnosed. Such a finding, if confirmed in other groups, would suggest that cat exposure in childhood is a risk factor for developing the disease.

Were NAMI members in the 1980s representative of families with a member with schizophrenia? At that

Funding source

The Stanley Medical Research Institute

Contributors

E. Fuller Torrey, MD Stanley Medical Research Institute, United States

Wendy Simmons, MA Stanley Medical Research Institute, United States

Robert H. Yolken, MD Stanley Laboratory of Developmental Neurovirology Johns Hopkins University, School of Medicine, United States

Conflict of interest

The authors have declared that there is no conflict of interest in relation to the subject of the study.

Acknowledgements

We are grateful to the NAMI families who provided us with the information.

References (11)

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